The localization of hydrophilic sites within an osmium polypyridyl compound can produce a negative activation energy for emission decay

The temperature dependence of the emission lifetimes of two structural isom ers of an osmium polypyridyl complex are reported. The compounds [Os(bpy)(2 )(4, 4'-dcbpy)] (1) and [Os(bpy)(2)(3, 5-dcbpy)] (2), where 4, 4'-dcbpy = 4 , 4'-dicarboxy-2, 2'-bipyridine and 3, 5-dcbpy = 3, 5-dicarboxy-2, 2'-bipyr idine. differ only in the placement of their two hydrophilic carboxylate gr oups within the unique bipyridine ligand. In argon-saturated water, the emi ssion life-time of 1 decreases with increasing temperature and can be fit t o the Arrhenius expression (k(obs) = k(0) exp(-E-a/RT)) to give values of k (0) = (2.6 +/- 0.3) x 10(8) s(-1) and E-a = +330 +/- 20 cm(-1). In contrast the emission of 2 measured in ethanol displays longer lifetimes at higher temperatures to give k(0) = (2.9 +/- 0.3) x 10(6) s(-1) and E-a = -590 +/- 20 cm(-1). Thus, the specific localization of the hydrophilic carboxylate g roups in these molecules significantly alters their photophysical propertie s and can produce negatively activated emission lifetimes. (C) 2001 Elsevie r Science B.V. All rights reserved.